The long-term goal of this project is to define the structural and functional properties that underly the integrative functions and general excitability of spinal cord interneurons involved in the control of locomotion and posture in mammals. Their pivotal role(s) in motor control are well recognized, as is the notion that changes in interneuron excitability are likely to be contributing factors in a variety of locomotor disturbances consequent to disease or trauma. But the details of interneuron structure and function are lacking, particularly concerning the mode of action of descending control systems and cellular input- output characteristics. These gaps are a significant impediment in studies of the effects of spinal injury and the cellular/network mechanisms that underlie the resulting functional impairments. Although different classes of interneurons share some common features, there is striking diversity in terms of synaptology, receptor and ion channel expression, dendritic structure, and axonal projections. It is our hypothesis that these differences are functionally meaningful, and as a consequence they provide different classes of cells with a broad repertoire of potential responses to injury. We propose a comprehensive study of the structural and functional architecture of identified interneurons at the cellular level in the intact (uninjured) spinal cord. We will test the hypotheses that 1) dendritic structure and synapse distribution are two of the key factors that determine the relative amount of synaptic current, from dendritic synaptic sites, that reaches the cell soma and controls cell firing (i.e. input-output properties), and 2) that different classes of ventral horn interneurons are differentially innervated by descending motor control systems and display unique patterns of synaptic organization. This integrated proposal will focus on various well-defined interneurons in the adult cat spinal cord and will employ powerful combinations of quantitative light and electron microscopical, immunocytochemical, electrophysiological, and computational approaches. The new insights to be gained on the significance of dendritic structure and synaptic distribution, and definition of the overall cellular properties and organization of interneurons in spinal cord circuits, will help to establish the essential baseline for understanding the effects of spinal cord injury on interneurons.